The present invention relates to a radio transmission apparatus and, more particularly, to a radio transmission apparatus for burst-transmitting a radio signal in time-division multiplex radio transmission.
A conventional radio transmission apparatus, as shown in FIG. 8, comprises a logic processing unit, 1, a modulation unit 2, and a high-frequency amplification unit 3.
The logic processing unit 1 includes a logic processing circuit 14 for performing differential conversion and a frequency divider 13. The logic processing unit 1 receives data 11 to be transmitted and an input clock signal 12 having a rate as an integer multiple of the data rate of the data 11.
The rate of the input clock signal 12 is, e.g., 8 times the data rate of the data 11 because the input clock signal 12 is used in a digital filter 21 connected to the output terminal of the logic processing circuit 14. The input clock signal 12 is divided into two components in the logic processing unit 1. One of the two components is transmitted to the modulation unit 2 connected to the output terminal of the logic processing unit 1, and the other is frequency-divided by the frequency divider 13 to have a rate equal to the data rate. In this example, a frequency-division ratio is 1/8. A frequency-divided clock 17 is used as a clock in the logic processing circuit 14. The logic processing circuit 14 extracts the timing signal of each time slot in time-division transmission.
The timing signal may be externally supplied. The timing signal is transmitted as a burst timing signal 15 to the high-frequency amplification unit 3 and the modulation unit 2 connected to the output terminal of the logic processing unit 1.
The modulation unit 2 includes a digital filter 21 and a modulation circuit 22. The digital filter 21 performs wave-shaping of a transmission spectrum in a baseband. For example, the digital filter 21 has a Gaussian filtering function, and converts a signal into a GMSK (Gaussian-filtered minimum shift keying) modulation signal.
The burst timing signal 15 resets the digital filter 21 at the beginning of each time slot, and is used for synchronizing data with a clock in the corresponding time slot.
The modulation circuit 22 modulates a carrier wave by using an output signal from the digital filter 21 as a modulation signal.
The high-frequency amplification unit 3 amplifies a modulated carrier wave 23 output from the modulation unit 2 and transmits it to an antenna 4. At this time, the waveforms of the leading and trailing edges of a burst-like signal are shaped to prevent the unnecessary spread of a spectrum at rise and fall times when the signal of each time slot is burst-transmitted. The burst timing signal 15 is used for adjusting the wave-shaping operations in the high-frequency amplification unit 3. This operation is shown in FIGS. 9A to 9C.
When the burst timing signal 15 shown in FIG. 9C is present for a burst-like data signal 16 shown in FIG. 9A, a high-frequency output signal 31 is smoothly wave-shaped at its leading and trailing edges, as shown in FIG. 9B. For this reason, the unnecessary spread of a spectrum caused by an abrupt change in amplitude of a transmission wave can be prevented.
In a conventional radio apparatus, however, a burst timing signal also resets the digital filter 21. For this reason, when the digital filter 21 is reset, an output from the digital filter 21 is discontinued or is not smoothly changed at a reset timing.
As shown in FIGS. 9A to 9C, assume that the burst timing signal 15 is positioned before the leading edge of the output signal 31, i.e., at a reset point. Even when discontinuity points are generated to spread the spectrum of the signal, the signal having the spread spectrum is not radiated from the antenna 4. Even if the signal is radiated, the level of the signal is very low, and the signal does not influence an adjacent frequency channel.
In a carrier wave transmission apparatus in which all time slots must be set in a transmission state as in the control channel of a base station of a mobile telephone system, signals are set in the states shown in FIGS. 10A to 10C. That is, as shown in FIG. 10A, time slots for transmitting the burst-like signals 16 are continuously used. For this reason, at a boundary between adjacent time slots, as shown in FIG. 10B, the digital filter 21 is reset by the burst timing signal 15 shown in FIG. 10C before the level of the immediately preceding high-frequency output signal 31 is sufficiently decreased.
For this reason, the high-frequency signal having a spectrum which is unnecessarily spread due to data discontinuity caused by the reset is radiated from the antenna 4. The high-frequency signal interferes with another communication using an adjacent frequency channel, and normal communication is not easily performed.